Method and apparatus for prediction and transform for small blocks

ABSTRACT

A method of video decoding for a video decoder includes determining whether a block size of a chroma block is less than or equal to a block size threshold. The method further includes, in response to a determination that the block size of the chroma block is greater than the block size threshold, selecting an intra prediction mode for the chroma block from a plurality of intra prediction modes. The method further includes, in response to a determination that the block size of the chroma block is less than or equal to the block size threshold, selecting the intra prediction mode for the chroma block from a subset of the plurality of intra prediction modes. The method further includes performing intra prediction for the chroma block based on a chroma sample obtained with the selected intra prediction mode to encode the chroma block.

INCORPORATION BY REFERENCE

This application is a continuation of U.S. patent application Ser. No.16/796,104, filed Feb. 20, 2020, which is a continuation of U.S. Pat.No. 10,609,402, issued on Mar. 31, 2020, and claims the benefit ofpriority to U.S. Provisional Application No. 62/665,858, “PREDICTION ANDTRANSFORM FOR SMALL BLOCKS” filed on May 2, 2018. The benefit ofpriority is claimed to each of the foregoing, and the entire contents ofeach of the foregoing are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GByte of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce aforementioned bandwidth or storage space requirements,in some cases by two orders of magnitude or more. Both lossless andlossy compression, as well as a combination thereof can be employed.Lossless compression refers to techniques where an exact copy of theoriginal signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between theoriginal and reconstructed signal is small enough to make thereconstructed signal useful for the intended application. In the case ofvideo, lossy compression is widely employed. The amount of distortiontolerated depends on the application; for example, users of certainconsumer streaming applications may tolerate higher distortion thanusers of television contribution applications. The compression ratioachievable can reflect that: higher allowable/tolerable distortion canyield higher compression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used in as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer bitsare required at a given quantization step size to represent the blockafter entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring samples valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

The mapping of an intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode to codewords, to complex adaptive schemes involving mostprobably modes and similar techniques. In all cases, however, there canbe certain directions that are statistically less likely to occur invideo content than certain other directions. As the goal of videocompression is the reduction of redundancy, those less likely directionswill, in a well working video coding technology, be represented by alarger number of bits than more likely directions.

In some coding standards, a picture may be represented as a coding treeunit (CTU). The CTU may be split into coding units (CUs) by using aquadtree structure denoted as a coding tree to adapt to various localcharacteristics. The decision whether to code a picture area usinginter-picture (temporal) or intra-picture (spatial) prediction is madeat the CU level. Each CU can be further split into one, two, or fourprediction units (PUs) according to the PU splitting type. Inside onePU, the same prediction process is applied and the relevant informationis transmitted to the decoder on a PU basis. After obtaining theresidual block by applying the prediction process based on the PUsplitting type, a CU can be partitioned into transform units (TUs)according to another quadtree structure similar to the coding tree forthe CU.

The prediction process may be performed on variable block sizes.However, small blocks do not need the same number of intra predictiondirections as blocks having larger block sizes. Furthermore, theprocessing of a small block may create additional burdens for a hardwareimplementation for an encoder or decoder since the prediction of onesmall block has to be performed after the reconstruction of aneighboring block. Additionally, the processing of a small block maycreate additional burdens for a software implementation for an encoderor decoder due to the burden of performing a mode decision for the smallblock.

SUMMARY

An exemplary embodiment of the present disclosure includes a method ofvideo decoding for a video decoder. The method includes determiningwhether a block size of a chroma block is less than or equal to a blocksize threshold. The method further includes, in response to adetermination that the block size of the chroma block is greater thanthe block size threshold, selecting an intra prediction mode for thechroma block from a plurality of intra prediction modes. The methodfurther includes, in response to a determination that the block size ofthe chroma block is less than or equal to the block size threshold,selecting the intra prediction mode for the chroma block from a subsetof the plurality of intra prediction modes. The method further includesperforming intra prediction for the chroma block based on a chromasample obtained with the selected intra prediction mode.

An exemplary embodiment of the present disclosure includes a videodecoder for video decoding. The video decoder includes processingcircuitry configured to determine whether a block size of a chroma blockis less than or equal to a block size threshold. The processingcircuitry is further configured to, in response to a determination thatthe block size of the chroma block is greater than the block sizethreshold, select an intra prediction mode for the chroma block from aplurality of intra prediction modes. The processing circuitry is furtherconfigured to, in response to a determination that the block size of thechroma block is less than or equal to the block size threshold, selectthe intra prediction mode for the chroma block from a subset of theplurality of intra prediction modes. The processing circuitry is furtherconfigured to perform intra prediction for the chroma block based on achroma sample obtained with the selected intra prediction mode.

An exemplary embodiment of the present disclosure includes anon-transitory computer readable medium having instructions storedtherein, which when executed by a processor in a video decoder causesthe processor to execute a method. The method includes determiningwhether a block size of a chroma block is less than or equal to a blocksize threshold. The method further includes, in response to adetermination that the block size of the chroma block is greater thanthe block size threshold, selecting an intra prediction mode for thechroma block from a plurality of intra prediction modes. The methodfurther includes, in response to a determination that the block size ofthe chroma block is less than or equal to the block size threshold,selecting the intra prediction mode for the chroma block from a subsetof the plurality of intra prediction modes. The method further includesperforming intra prediction for the chroma block based on a chromasample obtained with the selected intra prediction mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a simplified block diagram of acommunication system (100) in accordance with an embodiment.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 5 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 6 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 7 is a schematic illustration of exemplary intra prediction modes.

FIGS. 8A and 8B illustrate the various angular modes for 35 predictionmodes.

FIGS. 9A and 9B illustrate the various angular modes for 67 predictionmodes.

FIGS. 10A and 10B illustrates exemplary locations of samples used forderivation of a cross-component linear model (CCLM) prediction mode.

FIG. 11 illustrates an embodiment of a process performed by an encoderor decoder.

FIG. 12 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a simplified block diagram of a communication system(100) according to an embodiment of the present disclosure. Thecommunication system (100) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (150). Forexample, the communication system (100) includes a first pair ofterminal devices (110) and (120) interconnected via the network (150).In the FIG. 1 example, the first pair of terminal devices (110) and(120) performs unidirectional transmission of data. For example, theterminal device (110) may code video data (e.g., a stream of videopictures that are captured by the terminal device (110)) fortransmission to the other terminal device (120) via the network (150).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (120) may receive the codedvideo data from the network (150), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (100) includes a secondpair of terminal devices (130) and (140) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (130) and (140)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (130) and (140) via the network (150). Eachterminal device of the terminal devices (130) and (140) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (130) and (140), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 1 example, the terminal devices (110), (120), (130) and(140) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (150) represents any number ofnetworks that convey coded video data among the terminal devices (110),(120), (130) and (140), including for example wireline (wired) and/orwireless communication networks. The communication network (150) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(150) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 2 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (213), that caninclude a video source (201), for example a digital camera, creating forexample a stream of video pictures (202) that are uncompressed. In anexample, the stream of video pictures (202) includes samples that aretaken by the digital camera. The stream of video pictures (202),depicted as a bold line to emphasize a high data volume when compared toencoded video data (204) (or coded video bitstreams), can be processedby an electronic device (220) that includes a video encoder (203)coupled to the video source (201). The video encoder (203) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (204) (or encoded video bitstream (204)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (202), can be stored on a streamingserver (205) for future use. One or more streaming client subsystems,such as client subsystems (206) and (208) in FIG. 2 can access thestreaming server (205) to retrieve copies (207) and (209) of the encodedvideo data (204). A client subsystem (206) can include a video decoder(210), for example, in an electronic device (230). The video decoder(210) decodes the incoming copy (207) of the encoded video data andcreates an outgoing stream of video pictures (211) that can be renderedon a display (212)(e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (204),(207), and (209) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Codingor VVC. The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (220) and (230) can includeother components (not shown). For example, the electronic device (220)can include a video decoder (not shown) and the electronic device (230)can include a video encoder (not shown) as well.

FIG. (3) shows a block diagram of a video decoder (310) according to anembodiment of the present disclosure. The video decoder (310) can beincluded in an electronic device (330). The electronic device (330) caninclude a receiver (331) (e.g., receiving circuitry). The video decoder(310) can be used in the place of the video decoder (210) in the FIG. 2example.

The receiver (331) may receive one or more coded video sequences to bedecoded by the video decoder (310); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (301), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (331) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (331) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (315) may be coupled inbetween the receiver (331 and an entropy decoder/parser (320) (“parser(320)” henceforth). In certain applications, the buffer memory (315) ispart of the video decoder (310). In others, it can be outside of thevideo decoder (310) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (310), forexample to combat network jitter, and in addition another buffer memory(315) inside the video decoder (310), for example to handle playouttiming. When the receiver (331) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (315) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (315) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (310).

The video decoder (310) may include the parser (320) to reconstructsymbols (321) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (310),and potentially information to control a rendering device such as arender device (312) (e.g., a display screen) that is not an integralpart of the electronic device (330) but can be coupled to the electronicdevice (330), as was shown in FIG. 3. The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (320) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (320) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (320) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (320) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer memory (315), so as to createsymbols (321).

Reconstruction of the symbols (321) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (320). The flow of such subgroup control information between theparser (320) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (310)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (351). Thescaler/inverse transform unit (351) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (321) from the parser (320). The scaler/inversetransform unit (351) can output blocks comprising sample values, thatcan be input into aggregator (355).

In some cases, the output samples of the scaler/inverse transform (351)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (352). In some cases, the intra pictureprediction unit (352) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (358). The currentpicture buffer (358) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(355), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (352) has generated to the outputsample information as provided by the scaler/inverse transform unit(351).

In other cases, the output samples of the scaler/inverse transform unit(351) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (353) canaccess reference picture memory (357) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (321) pertaining to the block, these samples can beadded by the aggregator (355) to the output of the scaler/inversetransform unit (351) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (357) from where themotion compensation prediction unit (353) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (353) in the form of symbols (321) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (357) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (355) can be subject to variousloop filtering techniques in the loop filter unit (356). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (356) as symbols (321) from the parser (320), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (356) can be a sample stream that canbe output to the render device (312) as well as stored in the referencepicture memory (357) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (320)), the current picture buffer (358) can becomea part of the reference picture memory (357), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (310) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as document in thevideo compression technology or standard. Specifically, a profile canselect a certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (331) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (310) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 4 shows a block diagram of a video encoder (403) according to anembodiment of the present disclosure. The video encoder (403) isincluded in an electronic device (420). The electronic device (420)includes a transmitter (440)(e.g., transmitting circuitry). The videoencoder (403) can be used in the place of the video encoder (203) in theFIG. 2 example.

The video encoder (403) may receive video samples from a video source(401)(that is not part of the electronic device (420) in the FIG. 4example) that may capture video image(s) to be coded by the videoencoder (403). In another example, the video source (401) is a part ofthe electronic device (420).

The video source (401) may provide the source video sequence to be codedby the video encoder (403) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . )and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (401) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (401) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focusses on samples.

According to an embodiment, the video encoder (403) may code andcompress the pictures of the source video sequence into a coded videosequence (443) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (450). In some embodiments, the controller(450) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (450) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (450) can be configured to have other suitablefunctions that pertain to the video encoder (403) optimized for acertain system design.

In some embodiments, the video encoder (403) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (430) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (433)embedded in the video encoder (403). The decoder (433) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (434). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (434) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (433) can be the same as of a“remote” decoder, such as the video decoder (310), which has alreadybeen described in detail above in conjunction with FIG. 3. Brieflyreferring also to FIG. 3, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (445) and the parser (320) can be lossless, the entropy decodingparts of the video decoder (310), including the buffer memory (315), andparser (320) may not be fully implemented in the local decoder (433).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (430) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (432) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (433) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (430). Operations of the coding engine (432) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 4), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (433) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (434). In this manner, the video encoder(403) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (435) may perform prediction searches for the codingengine (432). That is, for a new picture to be coded, the predictor(435) may search the reference picture memory (434) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(435) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (435), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (434).

The controller (450) may manage coding operations of the source coder(430), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (445). The entropy coder (445)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (440) may buffer the coded video sequence(s) as createdby the entropy coder (445) to prepare for transmission via acommunication channel (460), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(440) may merge coded video data from the video coder (403) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (450) may manage operation of the video encoder (403).During coding, the controller (450) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of Intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (403) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (403) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (440) may transmit additional datawith the encoded video. The source coder (430) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to Intra prediction) makes uses of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first and a second reference picture thatare both prior in decoding order to the current picture in the video(but may be in the past and future, respectively, in display order) areused. A block in the current picture can be coded by a first motionvector that points to a first reference block in the first referencepicture, and a second motion vector that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency. In the merge mode, a block inthe current picture can inherit motion vectors of a neighboring block(e.g., that shares a boundary with the block, is disposed in a largerpartition region with the block) in the current picture.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed ina unit of blocks. For example, according to the HEVC standard, a picturein a sequence of video pictures is partitioned into coding tree units(CTU) for compression, the CTLUs in a picture have the same size, suchas 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, 4 CUs of 32×32 pixels, or 16 CUs of16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB) and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels and the like.

FIG. 5 shows a diagram of a video encoder (503) according to anotherembodiment of the disclosure. The video encoder (503) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (503) is used in theplace of the video encoder (203) in the FIG. 2 example.

In an HEVC example, the video encoder (503) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (503) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (503) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(503) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (503) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 5 example, the video encoder (503) includes the interencoder (530), an intra encoder (522), a residue calculator (523), aswitch (526), a residue encoder (524), a general controller (521) and anentropy encoder (525) coupled together as shown in FIG. 5.

The inter encoder (530) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (522) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform and, in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (522) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (521) is configured to determine general controldata and control other components of the video encoder (503) based onthe general control data. In an example, the general controller (521)determines the mode of the block, and provides a control signal to theswitch (526) based on the mode. For example, when the mode is the intra,the general controller (521) controls the switch (526) to select theintra mode result for use by the residue calculator (523), and controlsthe entropy encoder (525) to select the intra prediction information andinclude the intra prediction information in the bitstream; and when themode is the inter mode, the general controller (521) controls the switch(526) to select the inter prediction result for use by the residuecalculator (523), and controls the entropy encoder (525) to select theinter prediction information and include the inter predictioninformation in the bitstream. In various embodiments, the video encoder(503) also includes a residue decoder (528). The residue decoder (528)is configured to perform inverse-transform, and generate the decodedresidue data. The decoded residue data can be suitably used by the intraencoder (522) and the inter encoder (530). For example, the interencoder (530) can generate decoded blocks based on the decoded residuedata and inter prediction information, and the intra encoder (522) cangenerate decoded blocks based on the decoded residue data and the intraprediction information. The decoded blocks are suitably processed togenerate decoded pictures and the decoded pictures can be buffered in amemory circuit (not shown) and used as reference pictures in someexamples.

The residue calculator (523) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (522) or the inter encoder (530). Theresidue encoder (524) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (524) is configured to convert the residuedata in the frequency domain, and generate the transform coefficients.The transform coefficients are then subject to quantization processingto obtain quantized transform coefficients.

The entropy encoder (525) is configured to format the bitstream toinclude the encoded block. The entropy encoder (525) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (525) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 6 shows a diagram of a video decoder (610) according to anotherembodiment of the disclosure. The video decoder (610) is configured toreceive a coded pictures that are part of a coded video sequence, anddecode the coded picture to generate a reconstructed picture. In anexample, the video decoder (610) is used in the place of the videodecoder (210) in the FIG. 2 example.

In the FIG. 6 example, the video decoder (610) includes an entropydecoder (671), an inter decoder (680), a residue decoder (673), areconstruction module (674), and an intra decoder (672) coupled togetheras shown in FIG. 6.

The entropy decoder (671) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example,intra, inter, b-predicted, the latter two in merge submode or anothersubmode), prediction information (such as, for example, intra predictioninformation or inter prediction information) that can identify certainsample or metadata that is used for prediction by the intra decoder(672) or the inter decoder (680) respectively residual information inthe form of, for example, quantized transform coefficients, and thelike. In an example, when the prediction mode is inter or bi-predictedmode, the inter prediction information is provided to the inter decoder(680); and when the prediction type is the intra prediction type, theintra prediction information is provided to the intra decoder (672). Theresidual information can be subject to inverse quantization and isprovided to the residue decoder (673).

The inter decoder (680) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (672) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (673) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (673) mayalso require certain control information (to include the QuantizerParameter QP), and that information may be provided by the entropydecoder (671) (datapath not depicted as this may be low volume controlinformation only).

The reconstruction module (674) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (673) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (203), (403) and (503), and thevideo decoders (210), (310) and (610) can be implemented using anysuitable technique. In an embodiment, the video encoders (203), (403)and (503), and the video decoders (210), (310) and (610) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (203), (403) and (403), and the videodecoders (210), (310) and (610) can be implemented using one or moreprocessors that execute software instructions.

FIG. 7 illustrates an embodiment of exemplary angular modes used forintra prediction. In FIG. 7, depicted in the lower right is an exampleof nine prediction directions, which may be from H.265's 35 possibleprediction directions. The point where the arrows converge (701)represents the sample being predicted. The arrows represent thedirection from which the sample is being predicted. For example, arrow(702) indicates that sample (701) is predicted from a sample or samplesto the upper right, at a 45 degree angle from the horizontal axis.Similarly, arrow (703) indicates that sample (701) is predicted from asample or samples to the lower left of sample (701), in a 22.5 degreeangle from the horizontal axis.

Still referring to FIG. 7, on the top left there is depicted a squareblock (704) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (704) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index), and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (704) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (704). Intra picture prediction can work bycopying reference sample values from the neighboring samples asappropriated by the signaled prediction direction. For example, assumethe coded video bitstream includes signaling that, for this block,indicates a prediction direction consistent with arrow (702)—that is,samples are predicted from a prediction sample or samples to the upperright, at a 45 degree angle from the horizontal. In that case, samplesS41, S32, S23, and S14 are predicted from the reference sample R05.Sample S44 is then predicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

FIGS. 8A and 8B illustrate an embodiment of 35 total intra predictionmodes including 33 angular modes. Mode 0 is the INTRA_PLANAR mode, andmode 1 is the INTRA_DC mode. Furthermore, Modes 2-34 represent the intraangular modes INTRA_ANGULAR2-INTRA_ANGULAR34. Mode 10 is the horizontalangular mode, and mode 26 is the vertical mode.

FIGS. 9A and 9B illustrate an embodiment of 67 total intra predictionmodes including 65 angular modes. Mode 0 is the INTRA_PLANAR mode, andmode 1 is the INTRA_DC mode. Furthermore, modes 2-66 represent intraangular modes INTRA_ANGULAR2-INTRA_ANGULAR66, respectively. Mode 18 isthe horizontal mode, and mode 50 is the vertical mode.

A PU may contain both a luma component and a chroma component. Accordingto some embodiments, when determining a prediction mode for the chromacomponent, the encoder or decoder selects a chroma prediction mode fromone of a planar mode (i.e., INTRA_PLANAR), DC mode (i.e., INTRA_DC),horizontal mode, vertical mode, and/or a direct copy of the intraprediction mode for the luma component (DM mode).

According to some embodiments, a cross-component linear model (CCLM)prediction mode may be used to reduce a cross-component redundancy, inwhich the chroma samples are predicted based on reconstructed lumasamples of the same CU by using a linear model as follows:

pred_(C)(i,j)=α·rec_(L)′(i,j)+β,  Eq. 1:

where pred_(C)(i, j) represents the predicted chroma samples in a CU,and rec_(L)′(i, j) represents the downsampled reconstructed luma samplesof the same CU.

Parameters α and β may be derived by minimizing a regression errorbetween the neighbouring reconstructed luma and chroma samples aroundthe current block as follows:

$\begin{matrix}{{\alpha = \frac{{N \cdot {\sum( {{L(n)} \cdot {C(n)}} )}} - {\sum{{L(n)} \cdot {\sum{C(n)}}}}}{{N \cdot {\sum( {{L(n)} \cdot {L(n)}} )}} - {\sum{{L(n)} \cdot {\sum{L(n)}}}}}},} & {{Eq}.\mspace{14mu} 2} \\{{\beta = \frac{{\sum{C(n)}} - {\alpha \cdot {\sum{L(n)}}}}{N}},} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

where L(n) represents the down-sampled top and left neighbouringreconstructed luma samples, C(n) represents the top and leftneighbouring reconstructed chroma samples, and the value of N is equalto twice of the minimum of a width and a height of the current chromacoding block.

For a coding block with a square shape, the above two equations may beapplied directly. For a non-square coding block, the neighbouringsamples of the longer boundary may be subsampled to have the same numberof samples as for the shorter boundary. FIGS. 10A and 108 show anexample of the location of the samples used for the derivation of α andβ.

According to some embodiments, the CCLM prediction mode includesprediction between two chroma components (i.e., the Cr component ispredicted from the Cb component). For example, instead of using thereconstructed sample signal, the CCLM Cb-to-Cr prediction is applied inthe residual domain. This prediction may be implemented by adding aweighted reconstructed Cb residual to the original Cr intra predictionto form the final Cr prediction as follows:

pred*_(Cr)(i,j)=pred_(Cr)(i,j)+α·resi′_(Cb) ^((i,j))  Eq. 4:

The scaling factor α may be derived in a similar way as in the CCLMluma-to-chroma prediction, where a difference is an addition of aregression cost relative to a default α value in the error function sothat the derived scaling factor is biased towards a default value of−0.5 as follows:

$\begin{matrix}{{\alpha = \frac{{N \cdot {\sum( {{{Cb}(n)} \cdot {{Cr}(n)}} )}} - {\sum{{{Cb}(n)} \cdot {\sum{{Cr}(n)}}}} + {\lambda \cdot ( {- 0.5} )}}{{N \cdot {\sum( {{{Cb}(n)} \cdot {{Cb}(n)}} )}} - {\sum{{{Cb}(n)} \cdot {\sum{{Cb}(n)}}}} + \lambda}},} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

where Cb(n) represents the neighbouring reconstructed Cb samples, Cr(n)represents the neighbouring reconstructed Cr samples, and λ is equal toΣ(Cb(n)·Cb(n))>>9.

According to some embodiments, the CCLM luma-to-chroma prediction modemay be added as one additional chroma intra prediction mode. At theencoder side, one more rate distortion (RD) cost check for the chromacomponents may be added for selecting the chroma intra prediction mode.When intra prediction modes other than the CCLM luma-to-chromaprediction mode is used for the chroma components of a CU, CCLM Cb-to-Crprediction may be used for the Cr component prediction.

According to some embodiments, the minimum luma block partition is 4×4,and the minimum chroma block partition becomes 2×4 and 2×2 for YUV 4:2:2and 4:2:0 formats, respectively, since luma and chroma blocks may sharethe same partition structure. As a result, an intra prediction modeneeds to be signaled for a 2×2, 2×N and N×2 chroma block, which may becostly, and the coding efficiency improvement may not justify theadditional cost of intra mode coding. The present embodiments providethe significantly advantageous features of reducing the cost of intracoding small blocks by reducing the number of intra prediction modesused for small blocks.

According to some embodiments, a block is identified as a small blockwhen a size of a block is less than or equal to a block size threshold.As an example, the block size threshold may be an N×Y block, where N andY are positive integers. For example, if N and Y are equal to 4,examples of small blocks include a 2×2 block, 2×4 block, and/or a 4×2block.

Another example of a small block includes 2×N and N×2 blocks where N isa positive integer. Another example of a small block includes 3×N andN×3 chroma blocks, where N is a positive integer. Another example of asmall block includes 4×N and N×4 chroma blocks, where N is a positiveinteger.

In some embodiments, the block size threshold is one of a block area,block width, and block height of a smallest luma intra coded block unitsize. In some embodiments, the block size threshold is set according toa number of pixels, which may be a predefined value (e.g., 16 or 32), orsignaled in a bitstream, such as in a sequence parameter set (SPS), apicture parameter set (PPS), or a slice header.

According to some embodiments, for small chroma intra coded blocks(e.g., 2×4 and 4×2 blocks), only a subset of the intra prediction modesinstead of all of the intra prediction modes that are applied for otherblock sizes is used. The subset of intra prediction modes may be one ormore, but not all, of the intra prediction modes that are applied forother block sizes. In one embodiment, the intra prediction mode appliedon small chroma intra coded blocks may be only the DM mode. In anotherembodiment, the intra prediction mode applied on small chroma intracoded blocks may be only the CCLM mode.

According to some embodiments, the intra prediction modes applied onsmall chroma intra coded blocks may be one of the DM mode, CCLM mode,and another intra prediction mode X. The intra prediction mode X may beone of the planar mode, DC mode, horizontal mode, and vertical mode.

According to some embodiments, a reduced set of intra prediction modesis used. For example, the reduced set of intra prediction modes mayinclude only the DM and the CCLM modes, where a chroma intra mode indexis binarized by 1 bit to indicate which mode is used. For example, whenthe chroma intra mode index is set to 0, the DM mode is used, and whenthe chroma intra mode index is set to 1, the CCLM mode is used.

For small chroma coding blocks, transform coefficient signaling isreduced. In one embodiment, only one DC coefficient is signaled for achroma block having a block size that is less than or equal to the blocksize threshold. In another embodiment, no transform coefficient issignaled for a chroma block having a block size that is less than orequal to the block size threshold, where a coded block flag (CBF) may beinferred as zero. The CBF may be used to indicate the presence ofnon-zero coefficients in a block. For example, a CBF set to 1 indicatesthat the small chroma coding block includes non-zero coefficients, andthe CBF set to 0 indicates that the small chroma coding block does notinclude non-zero coefficients.

FIG. 11 illustrates an embodiment of a process that may be performed byan encoder or decoder such as intra encoder 522 or intra decoder 672,respectively. The process may start at step S1100 where a block size ofa chroma block is determined. The process proceeds to step S1102 whereit is determined whether the determined block size of the chroma blockis less than or equal to the block size threshold. If the determinedblock size of the chroma block is less than or equal to the block sizethreshold, the chroma block is determined to be a small block. Forexample, if the height, width, or area of the chroma block is less thanor equal to the block size threshold, the chroma block is determined tobe a small block.

If the determined block size is greater than the block size threshold,the process proceeds from step S1102 to step S1104 where an intraprediction mode for the chroma block is selected from a plurality ofintra prediction modes. However, if the determined block size is lessthan or equal to the block size threshold (i.e., chroma block is a smallblock), the process proceeds from step S1102 to step S1106 where theintra prediction mode for the chroma block is selected from a subset ofthe plurality of intra prediction modes. As discussed above, the subsetincludes one or more, but not all, of the plurality of intra predictionmodes.

The process proceeds from S1104 and from S1106 to S1108 where intraprediction for the chroma block is performed based on a chroma sampleobtained with the selected intra prediction mode. The processillustrated in FIG. 11 is completed after step S1108.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 12 shows a computersystem (1200) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 12 for computer system (1200) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1200).

Computer system (1200) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1201), mouse (1202), trackpad (1203), touchscreen (1210), data-glove (not shown), joystick (1205), microphone(1206), scanner (1207), camera (1208).

Computer system (1200) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1210), data-glove (not shown), or joystick (1205), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1209), headphones(not depicted)), visual output devices (such as screens (1210) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1200) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1220) with CD/DVD or the like media (1221), thumb-drive (1222),removable hard drive or solid state drive (1223), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1200) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1249) (such as, for example USB ports of thecomputer system (1200)); others are commonly integrated into the core ofthe computer system (1200) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1200) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1240) of thecomputer system (1200).

The core (1240) can include one or more Central Processing Units (CPU)(1241), Graphics Processing Units (GPU) (1242), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1243), hardware accelerators for certain tasks (1244), and so forth.These devices, along with Read-only memory (ROM) (1245), Random-accessmemory (1246), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1247), may be connectedthrough a system bus (1248). In some computer systems, the system bus(1248) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1248),or through a peripheral bus (1249). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1241), GPUs (1242), FPGAs (1243), and accelerators (1244) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1245) or RAM (1246). Transitional data can be also be stored in RAM(1246), whereas permanent data can be stored for example, in theinternal mass storage (1247). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1241), GPU (1242), massstorage (1247), ROM (1245), RAM (1246), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1200), and specifically the core (1240) can providefunctionality as a result of processor(s)(including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1240) that are of non-transitorynature, such as core-internal mass storage (1247) or ROM (1245). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1240). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1240) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1246) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1244)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component Interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

(1) A method of video decoding for a video decoder including determiningwhether a block size of a chroma block is less than or equal to a blocksize threshold; in response to a determination that the block size ofthe chroma block is greater than the block size threshold, selecting anintra prediction mode for the chroma block from a plurality of intraprediction modes; in response to a determination that the block size ofthe chroma block is less than or equal to the block size threshold,selecting the intra prediction mode for the chroma block from a subsetof the plurality of intra prediction modes; and performing intraprediction for the chroma block based on a chroma sample obtained withthe selected intra prediction mode.

(2) The method of feature (1), in which the subset of the plurality ofintra prediction modes consists of a direct copy mode (DM) that reusesan intra prediction mode of a luma block associated with the chromablock.

(3) The method of feature (1) or (2), in which the subset of theplurality of intra prediction modes consists of a Cross CorrelationLinear Model (CCLM) mode that derives the chroma sample from a lumasample of a luma block associated with the chroma block, in which theluma sample is obtained in accordance with an intra prediction mode ofthe luma block.

(4) The method of any one of features (1)-(3), in which the subset ofthe plurality of intra prediction modes is one of (i) a direct copy mode(DM) that reuses an intra prediction mode of a luma block associatedwith the chroma block, and (ii) a Cross Correlation Linear Model (CCLM)mode that derives the chroma sample from a luma sample of the luma blockassociated with the chroma block, in which the luma sample is obtainedin accordance with the intra prediction mode of the luma block.

(5) The method of feature (4), in which a one bit indicator indicateswhich of the DM and the CCLM mode is used.

(6) The method of any one of features (1)-(5), in which the subset ofintra prediction modes is one of (i) a direct copy mode (DM) that reusesan intra prediction mode of a luma block associated with the chromablock, (ii) a Cross Correlation Linear Model (CCLM) mode that derivesthe chroma sample from a luma sample of the luma block associated withthe chroma block, and (iii) a group of intra prediction modes that isone of (a) a planar mode, (b) DC mode, (c) horizontal mode, and (d)vertical mode, and in which the luma sample is obtained in accordancewith the intra prediction mode of the luma block.

(7) The method of any one of features (1)-(6), in which only one DCcoefficient is signaled for the chroma block having a block size that isless than or equal to the block size threshold.

(8) The method of any one of features (1)-(7), in which no transformcoefficient is signaled for the chroma block having a block size that isless than or equal to the block size threshold.

(9) The method of any one of features (1)-(8), in which the block sizethreshold is N×Y, in which N is an integer greater than 2 and Y is aninteger greater than 2, and in which the block size of the chroma blockis less than or equal to the block size threshold in response to adetermination that a height of the chroma block is less than or equal toN or a width of the chroma block is less than or equal to Y.

(10) The method of any one of features (1)-(9), in which the block sizethreshold is one of a block area, block width, and block height of asmallest luma intra coded block unit size.

(11) The method of any one of features (1)-(10), in which the block sizethreshold and the block size of the chroma block is a number of pixels.

(12) A video decoder for video decoding including processing circuitryconfigured to determine whether a block size of a chromia block is lessthan or equal to a block size threshold; in response to a determinationthat the block size of the chroma block is greater than the block sizethreshold, select an intra prediction mode for the chroma block from aplurality of intra prediction modes; in response to a determination thatthe block size of the chroma block is less than or equal to the blocksize threshold, select the intra prediction mode for the chroma blockfrom a subset of the plurality of intra prediction modes; and performintra prediction for the chroma block based on a chroma sample obtainedwith the selected intra prediction mode.

(13) The video decoder of feature (12), in which the subset of theplurality of intra prediction modes consists of a direct copy mode (DM)that reuses an intra prediction mode of a luma block associated with thechroma block.

(14) The video decoder of feature (12) or (13), in which the subset ofthe plurality of intra prediction modes consists of a Cross CorrelationLinear Model (CCLM) mode that derives the chroma sample from a lumasample of a luma block associated with the chroma block, in which theluma sample is obtained in accordance with an intra prediction mode ofthe luma block.

(15) The video decoder of any one of features (12)-(14), in which thesubset of the plurality of intra prediction modes is one of (i) a directcopy mode (DM) that reuses an intra prediction mode of a luma blockassociated with the chroma block, and (ii) a Cross Correlation LinearModel (CCLM) mode that derives the chroma sample from a luma sample ofthe luma block associated with the chroma block, in which the lumasample is obtained in accordance with the intra prediction mode of theluma block.

(16) The video decoder of feature (15), in which a one bit indicatorindicates which of the DM and the CCLM mode is used.

(17) The video decoder of any one of features (12)-(16), in which thesubset of intra prediction modes is one of (i) a direct copy mode (DM)that reuses an intra prediction mode of a luma block associated with thechroma block, (ii) a Cross Correlation Linear Model (CCLM) mode thatderives the chroma sample from a luma sample of the luma blockassociated with the chroma block, and (iii) a group of intra predictionmodes that is one of (a) a planar mode, (b) DC mode, (c) horizontalmode, and (d) vertical mode, and in which the luma sample is obtained inaccordance with the intra prediction mode of the luma block.

(18) The video decoder of any one of features (12) (17), in which onlyone DC coefficient is signaled for the chroma block having a block sizethat is less than or equal to the block size threshold.

(19) The video decoder of any one of features (12)-(18), in which notransform coefficient is signaled for the chroma block having a blocksize that is less than or equal to the block size threshold.

(20) A non-transitory computer readable medium having instructionsstored therein, which when executed by a processor in a video decodercauses the processor to execute a method comprising determining whethera block size of a chroma block is less than or equal to a block sizethreshold; in response to a determination that the block size of thechroma block is greater than the block size threshold, selecting anintra prediction mode for the chroma block from a plurality of intraprediction modes; in response to a determination that the block size ofthe chroma block is less than or equal to the block size threshold,selecting the intra prediction mode for the chroma block from a subsetof the plurality of intra prediction modes; and performing intraprediction for the chroma block based on a chroma sample obtained withthe selected intra prediction mode.

What is claimed is:
 1. A method of video encoding for a video encoder,the method comprising: determining whether a block size of a chromablock is less than or equal to a block size threshold; in response to adetermination that the block size of the chroma block is greater thanthe block size threshold, selecting an intra prediction mode for thechroma block from a plurality of intra prediction modes; in response toa determination that the block size of the chroma block is less than orequal to the block size threshold, selecting the intra prediction modefor the chroma block from a subset of the plurality of intra predictionmodes that consists of non-angular intra prediction modes, the subset ofthe plurality of intra prediction modes including a direct copy modethat reuses an intra prediction mode of a luma block associated with thechroma block; and performing intra prediction for the chroma block basedon a chroma sample obtained with the selected intra prediction mode toencode the chroma block, wherein only one DC coefficient is signaled forthe chroma block having the block size that is less than or equal to theblock size threshold.
 2. The method of claim 1, wherein the subset ofthe plurality of intra prediction modes consists of the direct copy modeand a Cross Correlation Linear Model (CCLM) mode that derives the chromasample from a luma sample of the luma block associated with the chromablock, wherein the luma sample is obtained in accordance with the intraprediction mode of the luma block.
 3. The method of claim 2, wherein aone bit indicator indicates which of the direct copy mode and the CCLMmode is used.
 4. The method of claim 1, wherein the subset of theplurality of intra prediction modes consists of the direct copy mode andone of (a) a planar mode and (b) a DC mode, and wherein a luma sample ofthe luma block is obtained in accordance with the intra prediction modeof the luma block.
 5. The method of claim 1, wherein no transformcoefficient is signaled for the chroma block having a block size that isless than or equal to the block size threshold.
 6. The method of claim1, wherein the block size threshold is N×Y, wherein N is an integergreater than 2 and Y is an integer greater than 2, and wherein the blocksize of the chroma block is less than or equal to the block sizethreshold in response to a determination that a height of the chromablock is less than or equal to N or a width of the chroma block is lessthan or equal to Y.
 7. The method of claim 1, wherein the block sizethreshold is one of a block area, block width, and block height of asmallest luma intra coded block unit size.
 8. The method of claim 1,wherein the block size threshold and the block size of the chroma blockis a number of pixels.
 9. A video encoder for video encoding,comprising: processing circuitry configured to: determine whether ablock size of a chroma block is less than or equal to a block sizethreshold, in response to a determination that the block size of thechroma block is greater than the block size threshold, select an intraprediction mode for the chroma block from a plurality of intraprediction modes, in response to a determination that the block size ofthe chroma block is less than or equal to the block size threshold,select the intra prediction mode for the chroma block from a subset ofthe plurality of intra prediction modes that consists of non-angularintra prediction modes, the subset of the plurality of intra predictionmodes including a direct copy mode that reuses an intra prediction modeof a luma block associated with the chroma block, and perform intraprediction for the chroma block based on a chroma sample obtained withthe selected intra prediction mode to encode the chroma block, whereinonly one DC coefficient is signaled for the chroma block having theblock size that is less than or equal to the block size threshold. 10.The video encoder of claim 9, wherein the subset of the plurality ofintra prediction modes consists of the direct copy mode and a CrossCorrelation Linear Model (CCLM) mode that derives the chroma sample froma luma sample of the luma block associated with the chroma block,wherein the luma sample is obtained in accordance with the intraprediction mode of the luma block.
 11. The video encoder of claim 9,wherein a one bit indicator indicates which of the direct copy mode andthe CCLM mode is used.
 12. The video encoder of claim 9, wherein thesubset of the plurality of intra prediction modes consists of the directcopy mode and one of (a) a planar mode and (b) a DC mode, and wherein aluma sample of the luma block is obtained in accordance with the intraprediction mode of the luma block.
 13. The video encoder of claim 9,wherein no transform coefficient is signaled for the chroma block havinga block size that is less than or equal to the block size threshold. 14.The video encoder of claim 9, wherein the block size threshold is N×Y,wherein N is an integer greater than 2 and Y is an integer greater than2, and wherein the block size of the chroma block is less than or equalto the block size threshold in response to a determination that a heightof the chroma block is less than or equal to N or a width of the chromablock is less than or equal to Y.
 15. The video encoder of claim 9,wherein the block size threshold is one of a block area, block width,and block height of a smallest luma intra coded block unit size.
 16. Anon-transitory computer readable medium having instructions storedtherein, which when executed by a processor in a video encoder cause theprocessor to execute a method comprising: determining whether a blocksize of a chroma block is less than or equal to a block size threshold;in response to a determination that the block size of the chroma blockis greater than the block size threshold, selecting an intra predictionmode for the chroma block from a plurality of intra prediction modes; inresponse to a determination that the block size of the chroma block isless than or equal to the block size threshold, selecting the intraprediction mode for the chroma block from a subset of the plurality ofintra prediction modes that consists of non-angular intra predictionmodes, the subset of the plurality of intra prediction modes including adirect copy mode that reuses an intra prediction mode of a luma blockassociated with the chroma block; and performing intra prediction forthe chroma block based on a chroma sample obtained with the selectedintra prediction mode to encode a chroma block, wherein only one DCcoefficient is signaled for the chroma block having the block size thatis less than or equal to the block size threshold.
 17. Thenon-transitory computer readable medium according to claim 16, whereinthe subset of the plurality of intra prediction modes consists of thedirect copy mode and a Cross Correlation Linear Model (CCLM) mode thatderives the chroma sample from a luma sample of the luma blockassociated with the chroma block, wherein the luma sample is obtained inaccordance with the intra prediction mode of the luma block.
 18. Thenon-transitory computer readable medium according to claim 17, wherein aone bit indicator indicates which of the direct copy mode and the CCLMmode is used.
 19. The non-transitory computer readable medium accordingto claim 16, wherein the subset of the plurality of intra predictionmodes consists of the direct copy mode and one of (a) a planar mode and(b) a DC mode, and wherein a luma sample of the luma block is obtainedin accordance with the intra prediction mode of the luma block.
 20. Thenon-transitory computer readable medium according to claim 16, whereinno transform coefficient is signaled for the chroma block having a blocksize that is less than or equal to the block size threshold.